Pulse width modulation frequency converter

ABSTRACT

A multi-phase voltage-controlled PWM frequency converter, comprising at least one control unit ( 23 ), at least one rectifier bridge ( 20 ) designed to be connected to a multi-phase supply network (U U , U V , U W ), a direct-voltage intermediate circuit and at least one controlled inverter bridge ( 21 ) for feeding at least one multi-phase load ( 22 ) with an alternating voltage (U S , U R , U T ) of varying magnitude and frequency. The inverter bridge has pulse width modulation-controlled semiconductor switches (V 11 -V 16 ) and, in parallel with these, inverse-parallel connected diodes (D 11 -D 16 ). The rectifier bridge has fully controllable semiconductor switches (V 1 -V 6 ) and, in parallel with these, inverse-parallel connected diodes (D 1 -D 6 ). The control unit controls the fully controllable semiconductor switches of the rectifier bridge in such manner that, in the upper arm, the switch of the phase concerned conducts substantially as long as the instantaneous value of the network phase voltage (U U , U V , U W ) in question is the most positive, and in the lower arm the switch of the phase concerned conducts substantially as long as the instantaneous value of the network phase voltage (U U , U V , U W ) in question is the most negative. The rectifier bridge ( 20 ) is connected to the inverter bridge ( 21 ) directly without a direct-voltage capacitor unit acting as an intermediate energy storage, and the direct current produced by the inverter bridge has been arranged to flow directly into the supply network without limitation of current peak value by an inductor unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multi-phase voltage-controlled PulseWidth Modulation (PWM) frequency converter, comprising a control unit, arectifier bridge designed to be connected to a multi-phase supply line,a direct-voltage intermediate circuit and a controlled inverter bridgefor supplying a multi-phase alternating voltage into a multi-phase load.

2. Description of Background Art

Three-phase voltage-controlled PWM frequency converters have a rectifierbridge for rectifying the three-phase alternating voltage of a supplyline to produce a d.c. voltage for a direct-voltage intermediatecircuit, and an inverter bridge for the inversion of the intermediatecircuit direct voltage into a variable-frequency three-phase alternatingvoltage while power is flowing in the direction from the supply line toa load, such as a cage induction motor. A cage induction motor isgenerally used in many applications, e.g. pumps or fans. The inverterbridge is a full-wave bridge with pulse-width-modulation controlledsemiconductor switches and with diodes connected in inverse-parallelwith these. The rectifier bridge may be an uncontrolled full-wavebridge, in which case only diodes are used in it, or a controlled one,in which case it is provided with controlled semiconductor switches andwith diodes connected in inverse-parallel with them. In the case of acontrolled rectifier bridge, power may also flow in the direction fromthe load to the supply line, e.g. in situations where a motor is beingbraked. A known possibility for implementing a controlled rectifierbridge is a three-phase circuit as presented in U.S. Pat. No. 4,447,868,which allows power flow either from the a.c. circuit into the d.c.circuit or vice versa. According to the above-mentioned patent,conduction by the transistors of the rectifier bridge is so controlledthat the transistor in the upper arm of the phase with the highestsupply voltage instantaneous value and the transistor in the lower armof the phase with the lowest supply voltage instantaneous value areconducting.

Prior-art solutions aim at maintaining a constant voltage in thedirect-voltage intermediate circuit by using a high-capacitance d.c.capacitor as an intermediate energy storage. Prior-art solutions alsogenerally use a three-phase a.c. inductor unit or a single-phase d.c.inductor unit in conjunction with the rectifier bridge in order to limitsupply line current peaks.

The ratings of the capacitor unit are generally determined by thecapacitors' ability to withstand the electric current and voltageloading applied to them and the required service life in extremeconditions. To determine the electric loading, the components generatedby the rectifier and inverter circuits are generally first calculatedseparately and then summed quadratically. This is the procedure observedwhen the capacitor unit has a considerable capacitance, in which casethe circuits can be regarded as separate circuits and theirinstantaneous values have no effect on each other. From these startingpoints it follows that the capacitance of the capacitor unit becomesfairly large because the preferable capacitor type, the electrolyticcapacitor, has a relatively low current tolerance. On the other hand, alarge capacitance value is advantageous in respect of various regulationfunctions (e.g. stability of motor voltage, operation in brakingsituations, operation in the event of a mains failure).

Due to the large capacitor unit, the direct voltage is nearly constant.As seen from the direction of the supply line, this has the consequencethat, in order to limit the mains current peak values, a considerableamount of inductance is needed at some point along the current path. Atpresent, this inductance is most commonly placed before the rectifierbridge, so it will simultaneously protect the rectifier bridge againstsupply line overvoltage spikes. The rating of the current limitinginductor is usually e.g. such that, with nominal current, the voltageprevailing across the inductor equals about 3-5% of the supply voltage.

Prior-art filter components are bulky and expensive. Therefore, they area very great factor affecting the size and cost of a frequencyconverter.

SUMMARY AND OBJECTS OF THE INVENTION

The object of the present invention is to eliminate the drawbacks ofprior-art solutions and to achieve a control arrangement that will makeboth the capacitor acting as an energy storage in the direct-voltageintermediate circuit and the inductor used to limit supply line currentpeaks superfluous.

The control arrangement makes it possible to connect the rectifierbridge to the inverter bridge directly without a direct-voltagecapacitor unit acting as an intermediate energy storage, so that thedirect current produced by the inverter bridge will flow directly intothe supply line without current limitation by an inductor unit.

A multi-phase PWM frequency converter according to an embodiment of theinvention uses a rectifier bridge which has fully controllablesemiconductor switches and, in parallel with these, inverse-parallelconnected diodes, and in which a control unit controls the conduction ofthe fully controllable semiconductor switches of the rectifier bridge sothat the fully controllable semiconductor switch in the upper arm of thephase with the most positive supply voltage instantaneous value and thefully controllable semiconductor switch in the lower arm of the phasewith the most negative supply voltage instantaneous value arecontinuously conducting. Thus, regardless of its direction, theintermediate circuit current can flow freely into the supply line. Thisembodiment of the invention is characterized in that the frequencyconverter requires no large-capacitance capacitor unit acting as anenergy storage to smooth the intermediate circuit voltage, nolarge-inductance inductor unit to limit the peak values of supply linephase currents and no measurement of the supply line phase currents orof the direct current as in prior-art solutions.

A voltage-controlled multi-phase PWM frequency converter according to asecond embodiment of the invention having diodes in its rectifier bridgeis characterized in that the control unit produces the output voltagepulse pattern via the controllable semiconductor switches of theinverter bridge by a pre-determined method in such manner that,regardless of frequency and load, the output power factor remains abovea preset minimum value, with the result that only positive currentvalues appear in the intermediate circuit current. Therefore, thefrequency converter need not be provided with a large-capacitancecapacitor unit acting as an energy storage to smooth the intermediatecircuit voltage nor with a high-inductance inductor unit to limit thepeak values of the supply line phase currents.

The details of the features characteristic of the frequency converter ofthe invention are presented in the attached claims.

Although the PWM frequency converter of the invention requires nocapacitor for smoothing the intermediate circuit d.c. voltage and noinductor for limiting the peak values of the mains current, a capacitorwith a low capacitance value may still be used in order to limit thevoltage spikes produced in switching situations by the energy latent inthe stray inductances of the direct-voltage circuit. Similarly, a filterunit consisting of inductors with a low inductance value and capacitorswith a low capacitance value may be used on the supply line side tofilter off high-frequency harmonics from the supply current. However,these components have no essential importance in respect of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in detail by the aidof a few examples with reference to the attached drawings, wherein

FIG. 1 presents a voltage-controlled PWM frequency converter,

FIG. 2 illustrates the formation of current in the direct-voltageintermediate circuit,

FIG. 3 illustrates the unfiltered direct current as well as thealternating current and voltage,

FIG. 4 illustrates the direct current at small power factor values,

FIG. 5 illustrates the switching sequence of the phase switches in sinewave-triangular wave modulation,

FIG. 6 illustrates plane-triangular wave modulation,

FIG. 7 illustrates two-pulse modulation,

FIG. 8 illustrates the motor power factor with constant torque,

FIG. 9 illustrates the motor power factor with quadratic torque and alinear and optimized voltage,

FIG. 10 presents a second voltage-controlled PWM frequency converteraccording to the invention,

FIG. 11 illustrates the control of the rectifier bridge of a frequencyconverter as presented in FIG. 10 as well as its direct current and oneof the phase currents, and

FIG. 12 presents the control electronics.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1Single-quadrant Drives

FIG. 1 presents a three-phase voltage-controlled PWM frequency converterwhich comprises a rectifier bridge 10 for the rectification of athree-phase alternating voltage obtained from a supply line andcomprising phase voltages U_(U), U_(V), U_(W) so as to produce a d.c.intermediate circuit d.c. voltage U_(DC), and an inverter bridge 11 forthe inversion of the direct voltage of the d.c. intermediate circuit soas to produce a variable-frequency three-phase alternating voltageconsisting of phase voltages U_(R), U_(S), and U_(T). In such afrequency converter, power can only flow in the direction from thesupply line to the load 12 (a three-phase cage induction motor M). Theinverter bridge 11 is a full-wave bridge in which a control unit 13controls the phase switches of each phase via pulse width modulation.“Phase switch” refers to the switch formed by the fully controllablesemiconductor switches in the upper and lower arms of each phase (phaseR: V11, V14; phase S: V12, V15, phase T: V13, V16; with inverse-parallelconnected diodes D11-D16 in parallel with them). The rectifier bridge 10is an uncontrolled full-wave bridge, consisting of a diode bridge withdiodes D1-D6.

In previously known technology, the direct-voltage intermediate circuitis provided with a capacitor C_(DC) for filtering the direct voltage andan inductor unit L_(AC) at the input of the rectifier bridge 10 forlimiting mains current peaks. As demonstrated later on, in thearrangement according to the present invention, both C_(DC) and L_(AC)are superfluous and the rectifier bridge 10 is connected to the inverterbridge 11 directly without a d.c. capacitor unit acting as anintermediate energy storage and the direct current I_(DC) produced bythe inverter bridge flows directly into the supply line without currentlimitation by an inductor unit.

The direction of the d.c. intermediate circuit current I_(DC) producedby the inverter bridge 11 has an essential importance regarding the needfor filtering in single-quadrant drives. For the intermediate circuitcurrent produced by the inverter bridge, the following basic rules applywhen the positive direction of the currents is toward the motor 12:

When all the phase switches are in the same position, then I_(DC)=0

When one of the switches is in the high position and two others are inthe low position, then I_(DC)=the current of the phase with the phaseswitch in the high position, as a positive current.

When one of the switches is in the low position and two others are inthe high position, then I_(DC)=the current of the phase with the phaseswitch in the low position, as a negative current.

FIG. 2 presents an example of how the current I_(DC) is formed when itis assumed that the device is operated with a full voltage (outputvoltage containing 1 pulse/half-cycle), that the current is sinusoidaland the power factor cos φ=0.87.

In this situation, the current I_(DC) is always positive. Now, if thedirect-voltage intermediate circuit has no filtering capacitor at all,then the supply current, e.g. I_(U), consists directly of the d.c.current as in the example in FIG. 3. As there is no intermediate energystorage, the supply current peak value is limited to the magnitude ofthe direct current even without any inductances connected to therectifier bridge to limit the current.

As can be inferred from FIGS. 2 and 3, the direct current I_(DC) iscontinuously positive as long as the motor circuit cos φ≧0.5 (i.e.φ≦60°). If cos φ is smaller than this, which is the situation in thecase of small loads (FIG. 4), then the direct current is negative forpart of the time. To prevent the d.c. voltage U_(DC) from rising toomuch, it is necessary to add to the circuit a voltage clipper consistingof e.g. a series connection of a diode and a capacitor. A clipper with asmall capacitance may be needed even during situations of a fullpositive current, because when abrupt changes occur in the directcurrent, the energy stored in the supply line inductances has to bedischarged somewhere. The voltage of the clipper capacitor can bedischarged e.g. by using it to power the control unit power supply.

In the case of partial voltages, when the output voltage containsseveral pulses/half-cycle, the situation in respect of a considerationof the current of the direct-voltage intermediate circuit is somewhatmore complicated than in a full-voltage range as described in theprevious example. However, the basic rules 1.-3. stated above apply inall situations, so the modulation method has a decisive effect on theeventual form of the intermediate circuit current. In respect ofreducing the clipper circuit used to limit d.c. circuit voltage peaks,it will be advantageous if no negative d.c. pulses appear until the cosφ value is as small as possible. In the following passages, the directcurrent I_(DC) will be considered with reference to a few differentmodulation methods.

In sine wave-triangular wave modulation, which at present is commonlyused with partial voltages for controlling the phase switches, a sinewave R, S, T (FIG. 5) for each phase is compared with a commontriangular wave. When the sine wave has a higher value than thetriangular wave, the phase switch concerned is in the high position, andvice versa. On the basis of the example presented in the figure, it canbe seen that the switching sequence of the phase switches e.g. from the0-point of phase R onwards is as shown in Table 1:

TABLE 1 Range + → − − → + Direct current  0° . . . 30° S - R - T T - R -S S, T 30° . . . 90° S - T - R R - T - S R, S

The ‘direct current’ column in the table indicates those output phasesof whose currents there appear samples in the intermediate circuitcurrent during modulation. For instance, samples of the R-phase currentappear in the intermediate circuit current I_(DC) immediately after 30°,which means that when cos φ≦0.87, the intermediate circuit currentcontains negative pulses. Therefore, sine wave-triangular wavemodulation is not a good modulation method in the case of partialvoltages, because cage induction motors generally have a lower cos φvalue than this.

Plane-triangular wave modulation is another generally known modulationmethod for the control of phase switches. The modulation works e.g. withprincipal voltage pulse number 5 as illustrated in FIG. 6. As can beseen from the figure, the phase switch for phase R does not assume adifferent position than the other two switches until in the range of60°-120°, which means that negative pulses only appear in I_(DC) whenthe motor circuit cos φ≦0.5. Thus, with this modulation method, the sameresult is reached as in a full-voltage range. The disadvantage of thismodulation method is that it produces harmonics in the output current,which may appear at very low frequencies as irregular rotation of themotor.

it is possible to further extend the cos φ range where no negativepulses appear in the direct current, by applying in the case of partialvoltages a two-pulse modulation method as illustrated in FIG. 7, inwhich the principal voltage contains two pulses for each half-cycle. Asshown in the figure, the phase switch for phase R only assumes adifferent position than the other two phase switches after an angle60°+α dependent on the degree of modulation has been reached, i.e. whencos φ<0.5. Therefore, with this modulation method, it is possible tooperate with a positive direct current at quite low voltage and cos φvalues.

The cos φ of motors at the nominal point varies depending on the modeland motor output, typical values being about 0.7 . . . 0.9. For partialpowers, cos φ is smaller. Frequency has no very pronounced effect on cosφ while the load (torque) is more decisive, as indicated by the examplepresented in FIG. 8 about a motor in constant-torque operation.

In the case of small loads, the diminution of costs can be stemmed bylowering the motor voltage in accordance with a pre-calculated voltagecurve dependent on frequency and load. By this method, it is possible tokeep cos φ continuously e.g. above the limit of 0.5, which is criticalin respect of plane-triangular wave modulation. FIG. 9 presents anexample illustrating the behavior of cos φ in the case of a quadraticload torque T (pump and fan drives) and a linear (Ulin) and optimized(Uopt) voltage.

Among the most appropriate processes working in a single quadrant inwhich a frequency converter according to the second embodiment of theinvention without an energy storing capacitor in the direct-voltageintermediate circuit and without a current limiting inductor at thesupply is applicable are pump and fan drives, because in these thedirection of power flow is always towards the motor, the load is alwaysquadratic and the operating point is always at high frequencies, so thate.g. the fluctuation in the rotational speed of the motor produced byplane-triangular wave modulation at low frequencies is no problem.

EXAMPLE 2 Four-quadrant Drives

FIG. 10 presents a three-phase voltage-controlled PWM frequencyconverter according to the second embodiment of the invention, whichcomprises a rectifier bridge 20 for the rectification of a three-phasealternating voltage consisting of phase voltages U_(U), U_(V), U_(W) toproduce a d.c. intermediate circuit direct voltage U_(DC) and aninverter bridge 21 for the inversion of the direct voltage of theintermediate circuit to produce a variable-frequency three-phasealternating voltage consisting of phase voltages U_(R), U_(S), U_(T).The frequency converter feeds a three-phase induction motor (M) 22. Theinverter bridge 21 is a full-wave bridge in which a control unit 23controls via pulse width modulation the fully-controllable semiconductorswitches V11-V16 of each phase (phase R: V11, V14; phase S: V12, V15;and phase T: V13-V16), each switch being connected in inverse-parallelwith a diode D11-D16.

In this embodiment, too, the rectifier bridge 20 is connected to theinverter bridge 21 directly without a direct-voltage capacitor unitfunctioning as an intermediate energy storage, and the direct currentI_(DC) produced by the rectifier bridge is passed directly to the a.c.supply line without current limitation by an inductor unit.

To allow the elimination of filtering in the direct-voltage circuitwithout any limitation regarding the direction of intermediate circuitcurrent, it is necessary to use a rectifier bridge circuit that permitsthe flow of negative intermediate circuit current in the directiontoward the supply line.

Such a circuit for the rectifier bridge 20 is achieved by connecting afully controllable semiconductor component, e.g. an integrated GateBipolar Transistor (IGBT), V1-V6, in parallel with each rectifier bridgediode D1-D6. As illustrated in FIG. 11, the rules for their control areas follows:

The IGBT in the upper arm of the phase having the highest instantaneousvoltage value is conducting, and

The IGBT in the lower arm of the phase having the lowest instantaneousvoltage value is conducting.

In other words, if the diode connected in parallel with the IGBT isconducting when power is flowing toward the motor, then the IGBT is alsokept conducting. Because of the IGBTs, power can just as readily flow inthe other direction as well.

By further connecting between the supply line and the rectifier bridgee.g. an LC filter consisting of inductors and capacitors, high-frequencyharmonics of the main current can be filtered as illustrated in FIG. 11.

FIG. 12 presents an example of how the above-described rectifier bridgecontrol logic can be implemented using opto-isolators. In this context,only the circuit on the side of the light emitter diodes of theopto-isolators is described. The pulse amplifier circuit on the side ofthe light detectors, which generates the actual voltage and currentpulse needed for controlling the power semiconductors in accordance withthe signal obtained from the opto-isolator, can be implemented in manyknown ways and will not be described here.

The circuit illustrated in FIG. 12 is connected to the same supplyvoltage terminals U_(U), U_(V), U_(W) presented in FIG. 10 to which therectifier bridge 20 is also connected. The circuit comprises a diodebridge D21-D26, opto-isolator emitter diodes V1 c-V6 c connected inseries with the diodes, and a resistor R1 connected to thedirect-voltage terminals of the diode bridge. In the circuit, a currentdetermined by resistor R1 flows through those emitter diodes which,according to the control logic presented in FIG. 11, are to give acontrol command to the semiconductor switches V1-V6 corresponding tothem. For example, a current is flowing through emitter diode V1 c onlywhen phase voltage U_(U) is more positive than the other phase voltages.Thus, the semiconductor switch V1 corresponding to this diode willconduct at exactly the right instant.

This embodiment of the invention imposes no restrictions as to theapplications of the frequency converter.

It is to be noted that the implementation of the rectifier bridgecontrol logic does not require any measurement of mains current ordirect current as do circuits implemented according to prior-arttechnology.

The invention can also be utilized in applications in which there areseveral three-phase systems feeding a common d.c. voltage intermediatecircuit (e.g. 12-pulse and 18-pulse bridges), or in which there areseveral inverter bridges connected to the same direct-voltageintermediate circuit to feed several separate loads.

It is obvious to the person skilled in the art that the embodiments ofthe invention are not restricted to the examples presented above, butthat they can be varied within the scope of the following claims.Besides IGBTs, the fully controllable semiconductor switches used mayalso consist of other fully grid-controlled semiconductor switches, i.e.switches that can be turned on and off, such as transistors.

What is claimed is:
 1. A multi-phase voltage-controlled PWM frequencyconverter, comprising at least one control unit, at least one rectifierbridge designed to be connected to a multi-phase supply line, adirect-voltage intermediate circuit and at least one controlled inverterbridge for feeding at least one multi-phase load with an alternatingvoltage of varying magnitude and frequency, said inverter bridge havingpulse width modulation-controlled semiconductor switches and, inparallel with these, inverse-parallel connected diodes, said rectifierbridge having fully controllable semiconductor switches and, in parallelwith these, inverse-parallel connected diodes, and said control unitcontrolling the fully controllable semiconductor switches of therectifier bridge so that, in an upper arm, the switch of the phaseconducts substantially as long as an instantaneous value of the supplyline phase voltage in question is most positive, and in a lower arm theswitch of the phase conducts substantially as long as the instantaneousvalue of a supply line phase voltage in question is most negative,wherein the rectifier bridge is connected to the inverter bridgedirectly without a direct-voltage capacitor unit acting as anintermediate energy storage, and a direct current produced by theinverter bridge is arranged to flow directly into the supply linewithout a current peak value limitation by an inductor unit in a directcurrent (dc) side or an alternating current (ac) side of the rectifier.2. Apparatus as defined in claim 1, wherein the control unit is providedwith control devices for controlling the semiconductor switches of therectifier bridge, said control devices being connected to the sameterminals of the supply line phase voltages as the rectifier bridge. 3.Apparatus as defined in claim 2, wherein the control devices areopto-isolators, and that the control unit comprises a diode bridge,opto-isolator emitter diodes connected in series with the diode bridgediodes and a resistor connected to the direct-voltage terminals of thediode bridge, a current determined by said resistor flowing throughthose emitter diodes which, according to the control logic, are to givea control command to the rectifier bridge semiconductor switchescorresponding to them.
 4. A multi-phase voltage-controlled PWM frequencyconverter comprising: at least one control unit, at least oneuncontrolled rectifier bridge designed to be connected to a multi-phasesupply line, a direct-voltage intermediate circuit and at least onecontrolled inverter bridge (11) for feeding a multi-phase load with analternating voltage of varying magnitude and frequency, said inverterbridge having pulse-width-modulation controlled semiconductor switchesand, in parallel with these, inverse-parallel connected diodes, saidrectifier bridge having diodes, and said control unit generating anoutput voltage pulse pattern so that only positive current pulses appearin the direct-voltage intermediate circuit when an output power factoris above a preset minimum value, wherein the control unit controls theoutput voltage by means of the pulse-width-modulation controlledsemiconductor switches in such manner that, regardless of frequency andload, the output power factor remains above the preset minimum value,and the rectifier bridge being connected to the inverter bridge directlywithout a direct-voltage capacitor unit acting as an intermediate energystorage, and direct current produced by the inverter bridge is arrangedto flow directly into the supply line without a limitation of a currentpeak value by an inductor unit in a direct current (dc) side or analternating current (ac) side of the rectifier.
 5. The multi-phasevoltage-controlled PWM frequency converter according to claim 4, whereinthe preset minimum value is 0.5.
 6. The multi-phase voltage-controlledPWM frequency converter according to claim 4, wherein the preset minimumvalue is 0.5 or greater.